The present invention relates generally to communication systems. More specifically, the present invention relates to code division multiple access (CDMA) communication systems that transmit pseudo noise (PN) codes from base stations to subscriber units over a pilot channel.
Terrestrial communication systems have provided convenient wireless communications services for years. These services include, for example, cellular telephone services, paging, Internet access, and data transfer services, among others.
Code division multiple access (CDMA, e.g. IS-95) wireless communication systems have become particularly prevalent, because they have shown significant advantages over other radio spectrum utilization schemes, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) systems. In particular, the increased efficiency of CDMA systems is due to improved coding and modulation density, interference rejection, multipath tolerance, and reuse of the same spectrum in every communication cell. The format of CDMA communication signals also makes it extremely difficult to intercept calls, thereby ensuring greater privacy for callers and providing greater immunity against fraud.
FIG. 1 illustrates a simplified block diagram of a terrestrial CDMA system 100, in accordance with the prior art. Network 100 includes one or more base station antennas 102 coupled to base transceiver stations (BTS) 103. Each BTS 103 communicates, via antennas 102 and subscriber links 104, with subscriber units carried by mobile users 106. Essentially, the BTS modulates and demodulates the information exchanged on the subscriber links 104, and it converts signals to and from the format used over the subscriber links.
BTS 103 also are coupled to a mobile switching office (MSO) 110. MSO 110 includes a switch that interfaces the cellular network and a public switched telephone network (PSTN, not shown). Thus, when network data originates from or is destined for a PSTN, this data is routed through MSO 110. The connection between a BTS 103 and MSO 110 can be a direct connection (e.g., using fiber optic or telephone (e.g., T1) links 105), or the connection 108 can be chained through other BTS 103, via links 108.
The network 100 also includes an operations and maintenance center (OMC) 111, which is manned by a human operator who evaluates status and control messages received from MSO 110, BTS 103, and/or other network elements or external sources. These messages could indicate, for example, that a piece of network equipment has failed, and/or how the various pieces of network equipment are performing. Based on the received messages, the OMC operator may initiate changes to the network, requesting that various network elements alter their operations.
In a CDMA system, data from multiple subscribers is spread across the same portion of the frequency spectrum. Each subscriber unit""s baseband data signal is multiplied by a code sequence, called the xe2x80x9cspreading code,xe2x80x9d which has a much higher rate than the data. Spreading the data in this manner results in a much wider transmission spectrum than the spectrum of the baseband data signal, hence the technique is called xe2x80x9cspread spectrum.xe2x80x9d
A spreading code typically is a pseudo noise (PN) code that consists of a certain number of bits. Different length PN codes can be used for various purposes. One particular type of PN code is a xe2x80x9cshort code,xe2x80x9d which each BTS periodically modulates and transmits over the network""s pilot channel. A pilot channel frame is as long as the short code, and evenly spaced offsets from the start of each pilot channel frame are identified as xe2x80x9coffsets.xe2x80x9d Each BTS periodically starts transmitting the short code over the pilot channel at an offset that is different from the offsets at which other BTS start transmitting the short code. These different offsets are typically assigned by OMC 111 or MSO 110. During each frame of the continuous succession of pilot channel frames, all BTS should start transmitting the short code at their assigned offsets.
FIG. 2 illustrates a timing diagram showing multiple BTS beginning transmission of the short code at different offsets, in accordance with the prior art. In particular, timing diagrams 201-205 for short codes transmitted over the pilot channel for five BTS are shown. In addition, a timing diagram 206 is shown, which illustrates search windows 207, described below, within which a subscriber unit should search for the beginning of a particular BTS"" short code. In the example shown in FIG. 2, BTS1201 begins short code transmission at offset 1, BTS2202 transmits at offset 3, BTS3203 transmits at offset 5, BTS4204 transmits at offset 2, and BTS5205 transmits at offset 4.
Typically, many more offsets than are illustrated in FIG. 2 are available for the system""s BTS to begin transmitting the short code. For example, the IS-95 protocol specifies that 512 offsets are available within a pilot channel frame, and each offset is separated from the previous offset by 64 chips. This means that the short code is 512xc3x9764 chips in length.
Subscriber units monitor the pilot channel and attempt to synchronize with candidate BTS to which the subscriber units can hand off. FIG. 3 illustrates a flowchart of a method for a subscriber unit to handoff from one BTS to another, in accordance with the prior art. The method begins, in block 302, when each subscriber unit that is communicating on the network receives, in a control channel from the BTS with which it is currently synchronized, a handoff candidate list that identifies various alternate BTS to which the subscriber unit, theoretically, could hand off. The handoff candidate list also identifies the offset at which each candidate BTS"" short code is transmitted. Handoff candidate lists are transmitted periodically to the subscriber units, but FIG. 3 illustrates receipt of only a single handoff candidate list for ease of description.
In block 304, the subscriber unit monitors the pilot channel and attempts to synchronize with short codes transmitted by the candidate BTS""s. The subscriber unit knows the short code, and attempts to correlate the known short code with the pilot channel signal within a time window, referred to as a xe2x80x9csearch window,xe2x80x9d that corresponds to the offset for each candidate BTS. Because the distance between the subscriber unit and each BTS imposes path delays on the signal, the search window has a width that accounts for path delays resulting from a range of possible distances between the candidate BTS""s and the subscriber unit. This range typically is known for a terrestrial network, and thus the search window can be fixed by the system. Generally, the search window is selected to be at least as narrow as the offset length, so that a maximum number of offsets can be allocated to BTS""s, and so that the subscriber unit is able to search for a particular short code quickly.
For each handoff candidate for which the subscriber unit finds a high degree of correlation between the known short code and the received signal, the subscriber unit measures the power of the signal, in block 306. The subscriber unit determines, in block 308, whether the power is sufficiently greater than the power of the signal transmitted by the current BTS. If not, the procedure iterates as shown. If so, then the subscriber unit requests to be handed off to the candidate BTS, in block 310, and the method ends. After handoff, since the subscriber unit is synchronized to the new BTS, the subscriber unit can despread data received from the new BTS using another known PN code.
In order for a system to provide a maximum number of offsets (and, thus, candidate BTS""s), the search window must be at least as narrow as the offset length. A system that incorporates this limitation makes it impossible for a subscriber unit to synchronize with a BTS that is farther than a certain distance from the subscriber unit. This is because a short code transmitted by a BTS that is too far away will not reach the subscriber unit within the time spanned by the search window. Therefore, even if a particular BTS has the capacity to provide service to a subscriber unit, it cannot do so if it is too far from the subscriber unit.
One solution is to widen the search window for each candidate BTS. An unfortunate result of this solution, however, is that fewer offsets would be available to assign if the search window width exceeds the pilot channel offset width. In such a case, less than the maximum number of candidate BTS""s would be available. In addition, the subscriber unit would take longer to search for each short code.
What are needed are methods and apparatus that enable a maximum number of BTS candidates to be potentially available for a subscriber unit to hand off to. In addition, what are needed are methods and apparatus that enable a subscriber unit to synchronize to a BTS short code even if that BTS is farther from the subscriber unit than the predefined search window otherwise permits.